In the gas turbine industry, a common problem with structural turbine casings is distortion of the casing, i.e., out-of-roundness, caused by the response of the casing to various temperature and pressure conditions during turbine operation. Typical turbine and compressor housings are formed in upper and lower halves connected one to the other along a horizontal plane by vertical bolts extending through radially outwardly directed and enlarged flanges at the housing splitline. One reason for the casing to distort is that the mass of the splitline flange is large, causing it to respond thermally at a rate slower than the response time for the balance of the turbine housing. Coupled with this large mass is a large thermal gradient through the flange which causes the flange to pinch inwards due to thermally induced axial strain. Another cause of distortion is a result of internal casing pressure. Further, it will be appreciated that there is an offset between the centerline of the boltholes and the main portion of the turbine casing at the splitline flanges. Because of this offset, a moment is introduced by the hoop field stress transferred through the bolts, causing the splitlines to deflect radially inwardly.
Efforts to minimize or prevent distortion of casings have included the use of very large bolts at the splitline with the bolts being spaced significant distances apart. The material between the bolts is then removed in an attempt to reduce the strain at the horizontal flanges. While this method may minimize or eliminate distortion of the casings, it introduces other problems, for example, flange leakage, and also does not address the problem of distortion due to internal pressure. Another method of controlling distortion is to flare the horizontal flange and reduce the distance between the bolt centerline and the casing mean hoop line. This reduces the moment that is introduced by the splitline flanges but requires a significant addition of material at the splitline flange. This increase in material carries with it a large thermal mass that responds much slower than the rest of the turbine casing, hence introducing thermal stresses. This is also a very costly alternative due to the additional material and the significant machining involved, including the large counterbores required for a design of this type. The added mass also includes a large thermal gradient which causes the splitline flange to pinch radially inwards due to the strain.